Some subjects, human and animal, require artificial electrical stimulation for the treatment of various symptoms. Patients with heart arrhythmias or heart blocks may utilize implanted pacemakers or combination pacemaker/defibrillators. But also some patients may be artificially stimulated by insertion of catheters into the heart during procedures such as electrophysiological studies. In fact, modern medicine is now exploring many stimulation therapies for hearts, muscles, nerves, and the brain. Some therapies like AV pacing or resynchronization therapy involve the application of multiple electrical pulses in precisely timed patterns to achieve the desired results.
The invention disclosed herein is broadly applicable to the detection of stimulus pulses which are applied to one or more regions (internal or external) of a living body in order to cause muscle or nerve cells to respond in a desired fashion. Cardiac pulses are one example of such stimulus pulses, cardiac pacing pulses such as those produced by cardiac pacemakers. The method of detection of such pulses is described herein in the context of detecting cardiac pacing pulses, but the inventive method is broadly applicable to the detection of many other stimulus pulses having similar signal characteristics. Further, the embodiments of the invention disclosed are also related to cardiac pacing. The use of cardiac pacing for description and illustration is not intended to be limiting.
Interpretation of electrical signals in such situations can often be complicated by the presence of the stimulus artifacts with the physiological signals being studied. In cardiac pacing, effective pacing stimuli normally occur immediately before the heart muscle response, which is generally called a P-wave in the atria or an R-wave in the ventricles. However, pacing stimuli that are sub-threshold, meaning not energetic enough to capture the heart muscle, can occur in isolation with no following muscle electrical response, or at any position within a heart cycle including within any part of the P, QRS and T waves of a naturally-occurring heart cycle. This dyssynchrony of stimulus with response complicates signal interpretation and yet is of critical importance.
In the event that stimulus artifacts (pulses) are mistaken for physiological activity such as heartbeats, devices intended to count heart rate may err. For a bedside cardiac monitor, a worst case occurs when a patient's heart has stopped beating but a heart monitor continues to report a normal heart rate, counting only ineffective pacing artifacts. In such situations, it is possible that no alarm will be given.
Often when a cardiac-paced patient is supervised such as during a procedure or in an ICU, there is less confusion as to when a pacing pulse has occurred. However, during the analysis of recorded signals taking place away from a patient, it may be important to be able to detect pacemaker artifacts in situations in which some confusion may exist in the recorded electrical signals.
The term “pacing spike” is also commonly used to refer to a stimulus artifact or stimulus pulse. The word “spike” is somewhat misleading, however, in that modern cardiac stimuli have become more complicated in shape, having multiple polarities and durations within a single stimulus. Throughout this document, various terms are used when referring to stimulus pulses.
Special electrical circuits operating in the analog domain or the extremely high bandwidth digital domain are usually able to distinguish stimulus artifacts from physiological signals, but these circuits add dramatically to the complexity and cost of equipment and the amount of storage memory required for signals.
Past methods to detect stimulus artifacts are dependent on the difference in signal characteristics of pacing artifacts compared to physiologic signals. Cardiac pacing stimuli tend to be very brief compared to the action potentials of muscle cells which are either naturally occurring or occurring in response to the stimuli. Typical pace pulse stimuli may be about one millisecond (msec) in duration while a QRS complex may be on the order of 100 msec wide.
The dominant pre-existing strategy to rely on the high-frequency content of pace artifacts is reasonable when detection occurs in body surface ECG channels. However, the frequency content of intra-cardiac records, or electrograms (EGM) can be significantly higher, and in appearance certain portions of EGMs are more difficult to distinguish from stimulus artifacts.
Much existing literature relates to implanted devices like pacemakers where the detection of the stimulus is not required because the device itself generates the stimulus. For these devices the interesting issue is how to remove or blank the resulting stimulus artifact so that the implanted device can sense the heart response and determine whether the stimulus was effective or not.
A number of methods for detecting stimulus pulses are known. Among these are methods disclosed in the patent documents described below. U.S. Pat. No. 4,105,023 (Marchese et al.) entitled “Pacemaker Artifact Suppression in Coronary Monitoring” describes a method which uses positive and negative slew rate detectors combined with multiple one-shot timing circuits and coincidence detection circuitry to detect a pace stimulus artifact. The circuitry operates in the analog domain on a single channel body surface ECG signal without digitization.
U.S. Pat. No. 4,149,527 (Naylor et al.) entitled “Pacemaker Artifact Suppression in Coronary Monitoring” discloses a method which uses a rate-limit circuit to detect pace artifacts and selects the rate-limit value (a) to pass those signals having frequency characteristics commensurate with either the normal PQRST complex of an ECG signal or the myoelectric artifacts commonly referred to as muscle noise, and (b) to rate-limit for higher slew rates which are characteristic of the discharge pulse portion of a pacer signal artifact. The Naylor et al. system involves transistor circuitry operating in the analog domain on a single channel ECG signal.
U.S. Pat. No. 4,664,116 (Shaya et al.) “Pace Pulse Identification Apparatus” describes a high-pass filter and comparator circuit with a threshold sensitive to steady high-frequency noise operating in the analog domain on a single ECG channel. The user is warned that some false detections can occur when the electrical equipment causing the high-frequency noise is turned on or its state of operation is changed. When high-frequency noise is present, the circuit reduces its sensitivity to pace artifacts such that small artifacts may not be identified.
U.S. Pat. No. 7,471,977 (Zinser et al.) entitled “Method and System for Detecting Pace Pulses” discloses a system which detects pace pulses in electrocardiogram data by processing one or more sets of electrocardiogram data using a non-linear detection algorithm. The system uses differentiation, negative product accumulation and a cross-multiplicative combiner, requiring significantly more complex calculations than the present invention.
U.S. patent application Ser. No. 14/687,010 (Reinke et al.) entitled “Pace Pulse Detector for an Implantable Medical Device” discloses techniques for detecting pacing pulses in a sensed electrical signal from an implantable medical device (IMD) by analyzing a single sensed electrical signal to detect two types of pulses, one type having a first set of characteristics and the other type having a second set of characteristics. Operation of the IMD is adjusted to account for the detected pulses by, for example, blanking the sensed electrical signal or modifying a tachyarrhythmia detection algorithm. Two other contemporaneously-filed U.S. patent applications (Ser. Nos. 14/686,947 and 14/687,053) include disclosures similar to the '010 application.
In the field of physiology, it is known that physiologic signals conduct as ions crossing cellular membranes and further that the conduction rate of such signals is on the order of 1 meter per second (m/s) in muscle tissue and up to about 120 m/s in nerve tissue. However, stimulus pulses applied to living-body tissue conduct directly as electricity, moving through the body at a rate on the order of 10% of the speed of light. This very high conduction rate compared to the rate of physiologic signals means that stimulus pulses in multi-channel signals occur essentially coincidentally. The present invention exploits this coincidence as a way to discriminate between stimulus pulses and other types of signals in order to find these “needles-in-a-haystack” within captured electrical signals derived from living bodies.
It is therefore desirable to have an efficient, simple and reliable method to automatically recognize the features within electrical signals which indicate the occurrence of an artificial electrical stimulus even when that stimulus occurs coincidental to physiological signals and other noise signals.